The heart muscles of humans (and other creatures) periodically contract to force blood through the arteries. As a result, irregularly-shaped pressure pulses exist in these arteries and cause them to flex or oscillate. The base line pressure for these pulses is known as the diastolic pressure and the peak pressure for these pulses is known as the systolic pressure. A further pressure value, known as the "mean arterial pressure" (MAP), represents a time-weighted average of the blood pressure.
In the past, various techniques and devices have been used for measuring one or more of these blood pressure values. The most common method involves applying a, pressure cuff about the upper arm of a subject and inflating it so as to stop the flow of blood in the brachial artery. The pressure is then slowly relieved while a stethoscope is used on the distal portion of the artery to listen for pulsating sounds, known as Korotkoff sounds, that accompany the re-establishment of blood flow in the artery. As the pressure in the cuff is reduced further, the Korotkoff sounds eventually disappear. The cuff pressure at which the Korotkoff sounds first appear during deflation of the cuff is a measure of the systolic pressure and the pressure at which these sounds disappear is a measure of the diastolic pressure. This method of blood pressure detection is generally known as the auscultatory method.
Various devices are well known in the prior art for automatically performing blood pressure measurements by the auscultatory method. These devices employ a pump to automatically inflate a pressure cuff and a microphone to convert the Korotkoff sounds into electrical signals which are easily detected by various types of circuits. Other techniques have also been used to detect blood pressure from outside the subject's body, e.g., via Doppler shifts in ultrasonic waves reflected by the artery wall. In addition, there are intrusive devices that are inserted directly into the blood vessels for measurement of the pressure. However, the most commonly used method for measuring blood pressure, other than the auscultatory method, is the oscillometric method.
The oscillometric method is based on the fact that the pumping of blood through the arteries by the heart causes the arteries to flex. Even in the area adjacent to or within a pressure cuff applied to the arm of a human, these pressure variations exist. In fact, the pressure variations will pass from the artery through the arm of the human with attenuation and into the pressure cuff itself. While these pressure variations are small compared to the typical pressure applied by the cuff, they are nevertheless detectable by a transducer located to measure the pressure within the cuff. It has been found that these pulses, called "complexes", have a peak-to-peak amplitude which is minimal for applied cuff pressures above the systolic pressure and below the diastolic pressure. The amplitude of these complexes, however, rises to a maximum value. Physiologically, the cuff pressure at this maximum value approximates the Mean Arterial Pressure (MAP) of the subject. It has further been found that the complex amplitudes of cuff pressures equivalent to the systolic and diastolic pressures have a fixed relationship to the MAP. Thus, the oscillometric method is based on measurements of detected complex amplitudes at various cuff pressures.
As disclosed in U.S. Pat. Nos. 4,360,029 and 4,394,034 both entitled "Automatic Mean Blood Pressure Reading Device", automated blood pressure measuring devices operating according to the oscillometric method have been proposed in which the peak-to-peak amplitude of the pressure complexes are detected at various applied cuff pressures. The amplitudes of these complexes, as well as the applied cuff pressure, are stored together as the device automatically changes the cuff pressure over the range of interest. These peak-to-peak complex amplitudes define anoscillometric "envelope" and are evaluated to find the maximum value and its related cuff pressure, which is approximately equal to the MAP. The cuff pressure below the MAP value which produces a peak-to-peak complex amplitude having a certain fixed relationship to the maximum value is designated as the diastolic pressure. Likewise, the equivalent cuff pressure above the MAP value which results in complexes having an amplitude with a certain fixed relationship to that maximum value is designated as the systolic pressure. The relationships of systolic and diastolic pressures, respectively, to the maximum value, are empirically derived ratios which assume varying levels depending on the preferences of those of ordinary skill in the art. Generally, these pressures are calculated in the range of 40 to 80% of the maximum value.
The reliability and repeatability of these methods hinges on the ability to accurately determine the oscillation magnitudes of the complexes. There are several barriers to accurate and reliable oscillation magnitude determination. First, signal attenuation due to cuff compliance results in inaccuracies in blood pressure determination and may also add time to the taking and determination of the blood pressure when a weak or irregular pulse is present. Second, artifacts caused by patient motion and other effects are nearly always present. These artifacts are superimposed upon the desired oscillation signal, causing it to be distorted. Third, many of the properties of the desired oscillation signal are not consistent from patient to patient, or even from oscillation to oscillation for a given patient. One factor which affects the consistency of these.
The prior art methods which follow the oscillometric methods have employed a variety of schemes to improve their accuracy and reliability. Most often, the schemes involve artifact detection and rejection. Examples of artifact rejection algorithms can be seen for example in the U.S. Pat. Nos. 4,360,029 and 4,394,034 noted above (artifact rejection algorithms look at, inter alia, select parameters such as peak height or time rate of change of successive samples or series of samples) and in U.S. Pat. No. 4,546,775 entitled "Detection of Blood Pressure Complexes in Automated Vital Signs Monitors" (rejection is based upon signal slope that is uncharacteristic of the true complex). These techniques will accept only pulses with certain properties, such as specific rise times, or certain consistencies, such as a consistent time between oscillations. While these techniques may work well in some cases, they may fail in other cases. Such artifact rejection schemes tend not to work well with very old or very ill patients, as such properties or consistencies may simply not be present. In these cases, these prior methods can yield unreliable measurements of blood pressure or no measurement at all.
As stated earlier, the reliability and repeatability of non-invasive blood pressure methods hinge on the ability to accurately determine the oscillation magnitudes of the complexes. There are several barriers to accurate and reliable oscillation magnitude determination. First, cuff compliance may cause signal attenuation. The cuff generally comprises a flexible and adjustable material and, as a consequence, there is a problem of non-invasive blood pressure oscillometric signal attenuation due to cuff compliance. For example, although the cuff is limited from expanding inward due to the limb of the patient, it has a tendancy to expand or balloon outward (away from the patient) resulting in an increased volume of the bladder chamber and also an irregular surface on the side of the bladder chamber away from the patient. Signal attenuation results in inaccuracies in the blood pressure determination and also increases the time for taking the blood pressure when a weak or irregular pulse is present.
Also, artifacts caused by patient motion and other effects picked up by the cuff may be superimposed upon the desired oscillation signal, causing it to be distorted. In both manual and automated blood pressure monitoring systems it is imperative that the oscillation complexes are true representations of the subject's blood pressure. A problem with conventional blood pressure measuring devices is that subject movement, environmental conditions, and external artifact create noise and pressure fluctuations which interfere with the accurate measurement of oscillation complexes using an inflatable blood pressure cuff. In particular, noise is caused when movement of the subject's arm causes the inflatable blood pressure cuff to contact an external object, thereby compressing the cuff between the object and the subject's arm. Such noise and artifact are particularly problematic in an ambulance or in an emergency room environment. This contact and resulting compression on the cuff creates a noise pressure signal which interferes with accurate measurement and monitoring of the oscillation complexes. The noise will sometimes occur at the same time as an oscillation complex, and therefore, may be very difficult to discount as noise and may not be filtered out properly by an automated blood pressure monitoring system having a filtering mechanism. Depending on the severity of contact, such noise can be mistaken for an oscillation complex. At a minimum, noise caused when the inflatable cuff contacts external objects interferes with accurate blood pressure measurements.
Such artifacts can be reduced using fixed cylinders having a bladder that inflates toward the arm. These devices are generally fixed to a chair or table and allow a subject to insert his or her arm into the cylinder and then activate a device that inflates the bladder about the arm and automatically determines a blood pressure. These cylinder type blood pressure measuring devices employ the cylinder to hold the cuff in place and the cylinder is not adjustable to different size limbs. Since these device are not adjustable to individual arm sizes, they do not always produce accurate blood pressure measurements. Moreover, since the subject cannot move his or her arm, motion artifacts are minimized. However, a less restrictive approach to reduce artifact caused by cuff movement is desired.
Although the art of non-invasive blood pressure monitoring is well developed, there remain some problems inherent in this technology, particularly with providing a non-invasive blood pressure measuring device that improves the signal gain of the blood pressure cuff and reduces the effects of external artifacts. It is desired to address these problems by providing a non-invasive blood pressure cuff that uses conventional materials, has a design which would impose minimal discomfort to the patient, and is easy to use by the operator. It is also desired to provide a signal enhancing cuff apparatus which enables MAP and systolic and diastolic blood pressures to be more accurately measured. The present invention is intended to meet these and other needs in the art.